A thin-film transistor has conventionally been used in a flat panel display device such as a liquid crystal display device or an organic EL display device, for example. In recent years, a flat panel display device has been required to respond to increasing demands for a larger screen, a higher resolution, and a higher frame rate, etc. Some flat panel display devices use an oxide semiconductor layer that can achieve high mobility as a semiconductor layer of a thin-film transistor. In particular, an oxide semiconductor layer made of IGZO, for example, has gained attention as an oxide semiconductor layer satisfying the aforementioned demands as it can be formed in a large area under a relatively low temperature and consumes a small current.
Wet etching is generally used for patterning of such an oxide semiconductor layer. As a result of a high etching rate during wet etching, the shape of an end surface of the oxide semiconductor layer being formed cannot be controlled easily. Thus, the resultant end surface is not given a substantially inclined shape but becomes a vertical end surface. This may cause a phenomenon that is called a step disconnection where a covering layer such as an insulating film formed so as to cover the oxide semiconductor layer is disconnected at a position corresponding to the end surface of the oxide semiconductor layer.